JPWO1997049953A1 - Solid waste gasification and melting treatment method - Google Patents

Abstract

(57) [Abstract] This invention is a treatment method in which solid waste such as municipal solid waste, solidified fuel, and biomass waste is pyrolyzed and gasified in a fluidized bed gasifier (1) and then combusted at high temperatures in a melting furnace (9), with primary combustion in the fluidized bed section (4) of the gasifier at a temperature within 600±50°C, secondary combustion in the freeboard section (3) of the gasifier at a temperature within 725±75°C, and tertiary combustion in the melting furnace (9) at a temperature 50 to 100°C higher than the melting temperature of the ash. The oxygen ratio in the primary combustion (the ratio of the amount of oxygen supplied to the theoretical amount of oxygen for combustion) is 0.1 to 0.3, the oxygen ratio in the secondary combustion at 0.05 to 0.1, and the oxygen ratio in the tertiary combustion at 0.9 to 1.1, for a total oxygen ratio of approximately 1.3.

Description

[Detailed Description of the Invention] Solid Waste Gasification and Melting Treatment Method Technical Field The present invention relates to a treatment method for solid waste, such as municipal solid waste, solid fuel, slurry fuel, waste plastic, waste FRP, biomass waste, and shredder dust (from end-of-life vehicles, home appliances, etc.), or fuel derived from solid waste, by gasification and combustion.

Here, solid fuel refers to municipal solid waste (MSW) that has been crushed, sorted, and then compressed with the addition of quicklime. Slurried fuel (SWM) refers to municipal solid waste that has been crushed, slurried with water, and then hydrothermally decomposed under high pressure to produce an oil. FRP stands for fiber-reinforced plastic. Biomass waste includes water supply and sewage waste (impurities, sewage sludge), agricultural waste (rice husks, rice straw), forestry waste (sawdust, bark, thinnings), industrial waste (pulp chip dust), and construction waste.

Background Technology Currently, 75% of municipal solid waste is incinerated using fluidized bed incinerators or stoker furnaces. However, for the following reasons, there is a strong demand for new, environmentally friendly waste treatment technologies to replace incineration. Specifically, there has been a rapid increase in demand for ash melting to extend the life of landfills, detoxify ash, and effectively utilize it as civil engineering and construction materials.

We need to comply with stricter dioxin regulations that are expected in the near future.

The conventional approach of addressing issues such as dioxin decomposition and ash melting individually inevitably increases the construction and operating costs of the entire treatment facility. However, reducing exhaust gas volumes through low-oxygen combustion makes it possible to downsize the gas treatment facilities.

There is a growing trend to view waste as an energy source and to make the most of it for power generation and other purposes.

Given this background, new waste treatment systems incorporating gasification are currently under development. Among these, two leading-edge systems are a system using a vertical shaft furnace as a gasifier (hereinafter referred to as the S system) and a system using a rotary kiln (hereinafter referred to as the R system). Figure 3 shows the flow chart for the S system. In the figure, reference numeral 31 denotes a melting furnace, which contains a preliminary drying zone 32, a pyrolysis zone 33, and a combustion/melting zone 34. Reference numeral 35 denotes a dust collector, and reference numeral 36 denotes a combustion chamber. In Figure 3, a denotes waste, b denotes coke and limestone, c denotes oxygen-enriched air, d denotes slag, e denotes product gas, f denotes air, g denotes dust, h denotes combustion exhaust gas, and k denotes metals.

As shown in Figure 3, the melting furnace 31 contains a preliminary drying zone 32 (200-300°C), a pyrolysis zone 33 (300-1000°C), and a combustion/melting zone 34 (1500°C or higher), which are layered from top to bottom. Waste a is introduced into the upper part of the furnace and descends through the furnace while exchanging heat with gases rising from the lower zones. The generated gas e from the top of the melting furnace 31 passes through a dust collector 35 and is supplied to a combustion furnace 36 where it is combusted at approximately 900°C. The carbide produced in the pyrolysis zone 33, along with the charged coke and limestone b, descends into the melting/combustion zone 34 and is combusted at high temperatures by oxygen-enriched air c supplied through the tuyeres. Slag d and metal k, which have become molten due to the high temperature, are discharged from the bottom of the furnace.

Figure 4 shows the flow chart for the R system. In the figure, reference numeral 41 denotes a pyrolysis drum. Adjacent to the pyrolysis drum 41 are a cooler 42, a sorting facility 43, a crusher 44, and a silo 45. Reference numeral 46 denotes a gyratory melting furnace, and reference numeral 47 denotes a high-temperature air heater. In Figure 4, a denotes waste, f denotes air, i denotes char, j denotes non-combustible materials, e denotes product gas, d denotes slag, and h denotes combustion exhaust gas. After being crushed, waste a is fed into the pyrolysis drum 41, which is externally heated by high-temperature air f. While being agitated by rotation, the waste a is slowly pyrolyzed and gasified in an oxygen-free atmosphere at approximately 450°C. Product gas e exiting the pyrolysis drum 41 is directly fed to the downstream gyratory melting furnace 46.

Meanwhile, the solid char i and non-combustible materials j are removed from the pyrolysis drum 41, cooled in a cooler 42, and then separated into coarse non-combustible materials j and fine char i by sieving in a separation facility 43. The char i is finely pulverized in a crusher 44, stored in a silo 45, and then sent to a melting furnace 46 where it is combusted at a high temperature of approximately 1,300°C together with the product gas e from the pyrolysis drum 41. Molten slag d is discharged from the bottom of the melting furnace 46.

The problems with the two methods mentioned above are discussed below. The S-type shaft furnace creates a high-temperature melting zone (1700–1800°C) at the bottom of the furnace, necessitating the use of auxiliary fuels such as coke, which increases operating costs and carbon dioxide emissions. Furthermore, because the metals contained in the waste are melted, the recovered metals are alloyed. This makes it difficult to recycle the metals separately as ingots. Furthermore, this method uses a fixed-bed furnace, but because the waste is piled high in the furnace, even if attempts are made to uniformly distribute gas through the gaps between the waste, blow-by and localized uneven flow are unavoidable. This hinders stable operation and inevitably causes significant fluctuations in furnace pressure, gas generation rate, and gas composition.

On the other hand, R-type rotary furnaces rely on external heat, which has inefficient heat transfer, necessitating significantly larger furnaces, posing a challenge to scaling up. The resulting char is removed from the rotary furnace along with other non-combustible materials, cooled, and then separated and removed. It is then finely pulverized and stored in a hopper. Appropriate amounts are then cut off as needed, transported, and fed to the melting furnace. The increased complexity of the equipment required for char handling not only increases equipment costs but also hinders stable operation. Furthermore, the sensible heat contained in the char is lost through cooling and radiation, which is undesirable in terms of energy utilization.

In light of the above-mentioned circumstances, the present invention aims to provide a solid waste gasification and melting treatment method that does not require auxiliary fuels such as coke, does not increase carbon dioxide emissions, recovers metals such as iron, copper, and aluminum in a clean, unoxidized state, uses a compact furnace that can be easily scaled up, and does not require equipment for char pulverization or other processes.

Disclosure of the Invention To achieve the above-mentioned objectives, we further investigated the R-type system and devised a system using a fluidized-bed furnace as a gasifier (hereinafter referred to as the F-type system). Furthermore, through repeated experiments, we found that the best method is a three-stage combustion of waste materials in a gasifier and melter. That is, primary combustion occurs in the fluidized-bed section of the gasifier, followed by secondary combustion in the freeboard section of the gasifier and tertiary combustion in a downstream melter. The ash, converted into molten slag by the tertiary combustion at a high temperature of 1300°C, is continuously discharged from the bottom of the melter. Through extensive experimentation, we were able to establish the operating conditions for each component of the F-type system.

Regarding furnace temperatures, it was found that primary combustion in the fluidized bed section of the gasifier should be within 600±50°C, preferably 600±30°C, secondary combustion in the freeboard section should be within 725±75°C, and tertiary combustion in the melting furnace should be 50 to 100°C higher than the ash melting temperature. If the fluidized bed temperature of the gasifier falls below 500°C, the pyrolysis gasification reaction slows, suppressing gas generation and increasing the accumulation of undecomposed materials in the fluidized bed. As a result, the amount of combustible materials (i.e., gas, tar, and char) sent to the melting furnace decreases, making it difficult to maintain the melting furnace's combustion temperature.

On the other hand, it has been found that if the temperature of the fluidized bed portion of the gasifier is above 650°C, the gasification reaction becomes too rapid, and fluctuations in the supply of combustible material to the melting furnace lead to fluctuations in gas generation, adversely affecting combustion within the melting furnace. Metals contained in waste with melting points lower than the fluidized bed temperature can be removed from the bottom of the gasifier along with the fluidized bed material. In particular, if aluminum recovery is intended, the gasification temperature must be set below 650°C, since aluminum's melting point is 660°C.

If the gasification temperature is set above the melting point of aluminum, most of the aluminum will vaporize and be collected as ash after the melting furnace, but much of it will be oxidized and become worthless.

Secondary combustion is performed in the freeboard section of the gasifier. However, it was found that if temperatures exceed 800°C, the flue connecting the gasifier and melter becomes susceptible to blockage due to softening of ash caused by localized high-temperature combustion. This secondary combustion is intended to reduce the load of tertiary combustion in the melter and to maximize the use of freeboard space. The combustion temperature in the melter can be measured with a radiation thermometer, but slag flow was smooth when the temperature was set 50-100°C higher than the ash melting temperature, as previously measured. However, raising the combustion temperature in the melter more than 100°C higher than the ash melting temperature is undesirable, as it unnecessarily increases furnace material wear.

In the F-type gasifier, sand (e.g., silica sand) is used as the bed material, with an average particle size selected between 0.4 and 0.8 mm. A particle size below 0.4 mm results in a low throughput per hearth area, making it uneconomical. A particle size above 0.8 mm increases the superficial velocity of the gas blown up from the fluidized bed, hindering the introduction of waste from above. This means that fine or light waste particles are entrained in the gas rather than falling into the fluidized bed. This also increases the amount of char entrained in the gas. The gasifier in this F-type gasifier features a shallow fluidized bed, and its bed height (static bed height) is preferably approximately 1.0 to 1.5 times the inner diameter of the fluidized bed. The superficial velocity of the air or oxygen-containing gas supplied for primary combustion or fluidization is preferably approximately six times the u _mf (minimum fluidization velocity) of the bed material. Therefore, the required gas flow rate per square meter of hearth should be in the range of 400 to 1,700 ^Nm3 /hr. The heat required for pyrolysis and gasification of waste is provided quickly and efficiently by partial combustion of the waste and the heat transfer action of the bed material. This type of heat supply method is generally called an internally heated type, and it is clear that internally heated furnaces are superior to externally heated types in terms of compactness and thermal efficiency.

The char produced during the primary combustion pyrolysis gasification reaction is reduced in size by partial combustion and the agitation of the bed material. Using hard silica sand as the bed material further accelerates char comminution. Because char is porous, its apparent specific gravity is low, at 0.1–0.2. As a result, a fine char fluidized layer forms on top of the sand fluidized layer. If left unattended, the char fluidized layer will reach the top of the freeboard section, interfering with the feeding of waste to the gasifier. Because the waste gasifier inlet is located above the sand fluidized layer, even if the top of the freeboard is maintained at a negative pressure of approximately −20 mmAq, if the waste inlet is submerged in the char fluidized layer, positive pressure will be created near the inlet, creating the risk of combustible gas leaking to the outside through the feeder. This is undesirable from both safety and odor prevention standpoints. As a result of our investigations, we devised a method to promote char combustion and prevent char accumulation by supplying air or oxygen-containing gas directly above the sand fluidized bed (Japanese Patent Application No. 7-349428). Regarding the supply rate of air or oxygen-containing gas, we found that if the superficial velocity of all gases in the freeboard section is set to 0.7 m/sec or more, preferably 1.0 m/sec or more, fine char particles of 1 mm or less can be easily transported by airflow to the downstream melting furnace. The air or oxygen-containing gas supply position should be within 1000 mm above the surface of the sand fluidized bed. This allows the freeboard not only to simply classify the airflow, but also to fully function as a secondary combustion section, particularly promoting char combustion.

The purpose of tertiary combustion in the melting furnace is to complete the low-oxygen combustion of waste, to melt the ash into slag, and to decompose dioxins and dioxin derivatives.

Tertiary combustion is also carried out with the supply of oxygen-containing gas. As mentioned above, the combustion temperature in the melting furnace is set at 50-100°C above the ash melting temperature to ensure stable slag flow. However, it was found that complete decomposition of dioxins and dioxin derivatives requires high-temperature combustion above 1300°C for at least 0.5 seconds (sec). Since the F method does not have a high-temperature zone (1700-1800°C) like the S method, auxiliary fuels such as coke are not required. Depending on the waste quality and the desired gas characteristics, air, oxygen-enriched air, or oxygen can be appropriately selected for primary, secondary, and tertiary combustion. The melting furnace consists of a combustion chamber at the front and a slag separation section at the rear. It was also found that for efficient gas and slag separation, the upward gas velocity in the slag separation section must be kept below 6 m/sec.

The F method is intended to achieve low-oxygen combustion with a total oxygen ratio of approximately 1.3. Here, oxygen ratio refers to the ratio of the amount of oxygen used during combustion to the theoretical combustion amount. Experiments have shown that the oxygen ratios for primary, secondary, and tertiary combustion are 0.1-0.3 for primary combustion in the fluidized bed section of the gasifier, 0.05-0.1 for secondary combustion in the freeboard section, and 0.9-1.1 for tertiary combustion in the melting furnace. That is, combustion is performed with an oxygen ratio of 0.1-0.3 in the fluidized bed section of the fluidized bed gasifier, 0.05-0.1 in the freeboard section, and 0.9-1.1 in the melting furnace, resulting in a combined oxygen ratio of approximately 1.3 for both furnaces.

This F system combines a fluidized bed gasifier and a melting furnace (preferably a swirl-type melting furnace). The system's features include: Due to combustion with a low oxygen ratio of about 1.3, exhaust gas emissions are significantly reduced.

The exhaust gas and ash do not contain dioxins.

The ash is recovered as harmless slag that does not leach heavy metals, etc. This helps to extend the life of landfill sites and makes it possible to use it for civil engineering materials, etc.

The energy contained in the gas, tar, and char produced in the gasifier can be effectively used for ash melting, eliminating the need for auxiliary fuel and keeping operating costs low.

Because the system incorporates functions for dioxin decomposition and ash melting, the overall facility is more compact and construction costs are lower than adding these functions to a conventional incineration facility.

Metals such as iron, copper, and aluminum can be recovered in a clean, unoxidized state, making them suitable for recycling.

Based on the above, it was clear that the F method, which uses a fluidized-bed gasifier, is superior to the S method in that it is easier to operate, does not require auxiliary fuel such as coke, does not increase carbon dioxide emissions, and recovers metals such as iron, copper, and aluminum in a clean, unoxidized state. It is also superior to the R method in that the gasifier is compact, has no mechanical drives, is easily scaled up, and does not require any equipment for pulverizing char (carbide).

Some conventional incineration facilities do not have ash melting equipment or do not have ash melting equipment nearby, making it difficult to dispose of the furnace ash and other ash discharged from incinerators and waste heat boilers. By accepting the ash from such combustion facilities and melting it together with other solid waste, it can be effectively utilized as high-quality slag containing no residual waste.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing the configuration of an apparatus for carrying out the solid waste gasification and melting treatment method of the present invention. FIG. 2 is a diagram showing the proportions of various products produced by pyrolysis of waste at different treatment temperatures. FIG. 3 is a schematic diagram showing an example of a conventional waste treatment system. FIG. 4 is a schematic diagram showing another example of a conventional waste treatment system.

BEST MODE FOR CARRYING OUT THE INVENTION The following describes a specific embodiment of the solid waste gasification and melting treatment method of the present invention with reference to the drawings. Please note that the following embodiment is merely an example and is not intended to limit the technical scope of the present invention.

Figure 1 is a schematic diagram showing the configuration of an apparatus for carrying out the gasification and melting process of solid waste according to the present invention. In Figure 1, reference numeral 1 denotes a fluidized-bed gasifier, to which waste is supplied by a metering device 2. Formed within the fluidized-bed gasifier 1 are a freeboard 3, a fluidized bed 4, an air distribution plate 5, and an air chamber 6. A separator 7 and a bucket conveyor 8 are located adjacent to the fluidized-bed gasifier 1.

The gas, tar, and char produced in the fluidized-bed gasifier 1 are supplied to the downstream swirling melting furnace 9. The swirling melting furnace 9 contains a primary combustion chamber 10, a secondary combustion chamber 11, and a slag separation section 12. Reference numeral 13 denotes a water granulation trough, and reference numeral 14 denotes a slag conveyor. In Figure 1, a denotes waste, f denotes air, l denotes sand, j denotes non-combustible materials, g denotes product gas, d denotes slag, m denotes circulating water, and h denotes combustion exhaust gas.

After pre-processing such as crushing and sorting as necessary, waste a is supplied to a fluidized-bed gasifier 1 at a constant rate by a screw-type constant-rate feeder 2. Primary combustion air f is supplied to the air chamber 6 of the gasifier 1 and blown upward through the air distributor 5, fluidizing the sand l packed on the distributor. The sand used as the fluidizing medium is silica sand. Waste a supplied to the upper part of the fluidized bed falls into the bed and is rapidly pyrolyzed and gasified by contact with the primary combustion air f in the fluidized bed maintained at 600±50°C, preferably 600±30°C.

To prevent the accumulation of non-combustible materials in the fluidized bed, a mixture of non-combustible materials (j) and sand (l) is continuously or intermittently discharged from the bottom of the gasifier (1). Larger non-combustible materials (j) are separated and removed using a separator (7) such as a trommel. The non-combustible materials (j) contain metals such as iron, copper, and aluminum. However, because the furnace has a reducing atmosphere, these metals can be recovered in a clean, unoxidized state, polished by sand. This is an extremely important technology for recycling valuable metals. After the bulky non-combustible materials (j) have been removed, the sand (l) is transported upward by a bucket conveyor (8) and returned to the gasifier (1).

Waste a fed into the gasifier is rapidly converted into gas, tar, and char through pyrolysis and gasification. The char, a carbide, is dispersed within the fluidized bed and fluidized with the sand. It is gradually refined by oxidation or the turbulent motion of the fluidized bed. The refined char forms a fluidized bed above the fluidized bed. Secondary combustion air f is blown directly above the fluidized bed, and a second stage of combustion is carried out at a temperature within 725 ± 75°C. The refined char is entrained in the ascending gas flow. The char transported in this airflow exits the gasifier 1 along with the gas and tar and is fed into the primary combustion chamber 10 of the swirl-type melting furnace 9. It is mixed with preheated tertiary combustion air f in a powerful swirling flow and rapidly combusted at temperatures above 1300°C. Combustion is completed in the secondary combustion chamber 11, and the combustion exhaust gas l is discharged from the top of the slag separation section 12. Due to this high-temperature combustion, the ash contained in the char becomes slag mist. The slag d captured on the wall of the primary combustion chamber 10 by the centrifugal force of the swirling flow flows down the wall by gravity into the secondary combustion chamber 11, then from the secondary combustion chamber 11 to the slag separation section 12, and then from the bottom of the slag separation section 12 down to the granulation trough 13.

The granulation trough 13 resembles a slide over which water flows, and is positioned with the utmost care to prevent steam explosions even if a large lump of slag were to fall. The slag d that flows down the granulation trough 13 is rapidly cooled by contact with the circulating water m that flows rapidly over the trough, turning it into pea-shaped granulated slag and entering the slag conveyor 14. The granulated slag d is then transported to the exterior by the slag conveyor 14. The granulated slag d transported by the slag conveyor 14 is finely crushed into granules several millimeters in size during transport. The high-temperature combustion in the gyratory melting furnace 9 is intended to decompose dioxins and dioxin derivatives. The combined volume of the primary combustion chamber 10 and secondary combustion chamber 11 is designed to provide a gas residence time of at least 0.5 seconds. The combustion exhaust gas (h) leaving the swirling melting furnace (9) passes through a series of heat recovery devices, such as a waste heat boiler, a coal economizer, and an air preheater, as well as a dust removal device, before being released into the atmosphere. Each combustion chamber of the swirling melting furnace (9) is equipped with an oil burner for startup.

Figure 2 shows the ratio of various products depending on the processing temperature when pyrolyzing waste using the gasifier shown in Figure 1. Figure 2 confirms that secondary combustion at the freeboard temperature of the gasifier effectively increases gas yield. This is because the larger amount of gas is instantly combusted in the subsequent melting furnace, allowing for a smaller melting furnace.

As described above, this invention combines a fluidized-bed gasifier and a melting furnace. Primary combustion occurs in the fluidized-bed section of the gasifier, secondary combustion occurs in the freeboard section, and then high-temperature tertiary combustion occurs in the melting furnace. This melts the ash into slag and decomposes dioxins. This provides a next-generation waste treatment technology that excels in simplicity, compactness, material recycling, energy recycling, and environmental conservation, while also improving ease of operation and safety.

Industrial Applicability This invention is a technology for treating solid waste, such as municipal solid waste, solid fuel, slurry fuel, waste plastic, waste FRP, biomass waste, and shredder dust (from end-of-life vehicles, home appliances, etc.), or fuel derived from solid waste, and can be effectively used in waste treatment.

───────────────────────────────────────────────────── Continued from the front page (72) Inventor: Tetsuhisa Hirose 1-1 Haneda Asahi-cho, Ota-ku, Tokyo Ebara Corporation (72) Inventor: Takahiro Oshita 1-1 Haneda Asahi-cho, Ota-ku, Tokyo Ebara Corporation (Note) This publication is based on the publication published internationally by the International Bureau of Patents (WIPO). The effect of the international publication of the Japanese patent application (Japanese utility model registration application) related to this publication arises pursuant to Article 184-10, Paragraph 1 of the Patent Act (Article 48-13, Paragraph 2 of the Utility Model Act) and is unrelated to this publication.

Claims (6)

[Claims] 1. A solid waste gasification and melting treatment method in which solid waste is pyrolyzed and gasified in a fluidized-bed gasifier, followed by high-temperature combustion in a melting furnace, wherein primary combustion in the fluidized-bed section of the gasifier is conducted at a temperature within 600±50°C, secondary combustion in the freeboard section of the gasifier is conducted at a temperature within 725±75°C, and tertiary combustion in the melting furnace is conducted at a temperature 50 to 100°C higher than the ash melting temperature; the oxygen ratio (ratio of supplied oxygen to the theoretical amount of oxygen for combustion) in primary combustion is 0.1 to 0.3, the oxygen ratio in secondary combustion is 0.05 to 0.1, and the oxygen ratio in tertiary combustion is 0.9 to 1.1, resulting in a total oxygen ratio of approximately 1.3. 2. The method for gasifying and melting solid waste according to claim 1, wherein the supply rate of oxygen-containing gas used for primary combustion in the fluidized bed section of the gasifier is 400 to 1,700 ^Nm³ /hr per square meter of hearth. 3. The method for gasifying and melting solid waste according to claim 1, wherein the oxygen-containing gas used for secondary combustion in the freeboard section of the gasifier is supplied at a height of within 1,000 mm above the surface of the fluidized bed. 4. The method for gasifying and melting solid waste according to claim 1, wherein the superficial velocity of the gas in the freeboard section of the gasifier is 0.7 m/sec or more. 5. A method for gasifying and melting solid waste as set forth in claim 1, wherein the tertiary combustion in the melting furnace maintains high-temperature combustion at a temperature 50 to 100°C higher than the melting temperature of the ash for 0.5 seconds or more. 6. The method for gasifying and melting solid waste according to claim 1, wherein the superficial velocity of the combustion exhaust gas in the slag separation section of the melting furnace is set to 6 m/sec or less.